1. Field of the Invention
This invention relates to a (meth)acrylate, a polymer, a photoresist composition, and a pattern forming process making use of the composition. More specifically, the present invention is concerned with a photoresist composition and a pattern forming process, which are suitable for use in a lithographic step in the fabrication of a semiconductor device, especially in lithography making use of radiation of 220 nm or shorter in wavelength as exposure radiation.
2. Description of the Related Art
In the field of fabrication of various electronic devices requiring small geometry processing of the half micron order and led by semiconductor devices, there is an ever-increasing demand toward devices of still higher density and integration. This has led to still severer requirements for lithographic technology which is adopted for the formation of submicrometer patterns.
As one of measures for achieving miniaturization of a pattern, there is an approach to shorten the wavelength of exposure radiation which is used upon formation of a resist pattern. For a mass-fabrication process of 256 Mb DRAMs (processing dimension: xe2x89xa60.25 xcexcm), use of KrF excimer laser (wavelength: 248 nm) of shorter wavelength than i-line (wavelength: 365 nm) as an exposure radiation source in place of i-line is positively considered these days.
However, a radiation source of a still shorter wavelength is considered to be needed for the fabrication of DRAMs having an integration degree of 1 Gb or higher which requires still smaller geometry processing technology (processing dimension: xe2x89xa60.18 xcexcm). In particular, use of photolithography making use of ArF excimer laser (wavelength: 193 nm) has been reported recently (Donald C. Hoffer, et al., Journal of Photopolymer Science and Technology, 9(3), 387-397 (1996).
There is accordingly an outstanding desire for the development of a resist which can be successfully employed in photolithography making use of ArF radiation. Such an ArF exposure resist is required to achieve an improvement in the cost performance of laser because, inter alia, the gas life of the excimer laser light source is short and a laser apparatus itself is expensive.
In addition to such a high resolution as permitting a still further reduction in the processing dimension, the ArF resist is also required to exhibit still higher sensitivity. As a method for providing a resist with higher sensitivity, chemical amplification making use of a photoacid generator as a sensitizer is known widely. As a representative example, JP Kokoku 2-27660 discloses a resist which is composed, in combination, of triphenylsulfonium hexafluoroarsenate and poly(p-tert-butoxycarbonyloxy-xcex1-methylstyrene). Chemical modification resists are now extensively employed as KrF eximer laser resists [for example, Hiroshi Ito, C. Grantwillson, American Chemical Society Symposium Series 242, 11-23 (1984)]. A chemically amplified resist is characterized in that protonic acid, which is generated from a photoacid generator as a component of the resist upon exposure to radiation, undergoes an acid-catalyzed reaction with a resist resin or the like when subjected to a heat treatment after the exposure. As a result, chemically amplified resists have attained far higher sensitivity over conventional resists the photoreaction efficiencies (reactions per photon) of which are lower than 1. Nowadays, most of newly developed resists are of the chemically amplified type, and the adoption of a chemical amplification mechanism has become indispensable in the development of a high-sensitivity material which can meet the trend toward an exposure radiation source of shorter wavelength.
In lithography making use of radiation of a short wavelength of 220 nm or shorter led by ArF eximer laser, however, a resin component of a chemically amplified photoresist for use in the formation of submicrometer patterns is required to have new properties unsatisfiable by conventional materials, that is, high transparency for exposure light of 220 nm or shorter and resistance to dry etching.
In the conventional lithography which uses g-beam (438 nm), i-line (365 nm) or KrF eximer laser (248 nm), a resin having an aromatic ring in its structural units, such as a novolac resin or poly(p-vinylphenol), is used as a resin component of a photoresist composition, so that the resin is allowed to exhibit resistance to etching owing to the dry etching resistance of these aromatic rings. However, a resin containing aromatic rings shows extremely strong absorption for radiation of a wavelength shorter than 220 nm. Exposure radiation is therefore mostly absorbed at a resist surface and is unable to pass to a substrate, so that no submicrometer resist pattern can be formed. Conventional resins cannot accordingly be applied to photolithography which makes use of short-wavelength radiation of 220 nm or shorter. As a consequence, there is an outstanding strong desire for a resin material which does not contain aromatic rings, has etching resistance, and is transparent to wavelengths of 220 nm and shorter.
Proposed examples of high molecular compounds having transparency to ArF eximer laser (193 nm) and dry etching resistance include a copolymer containing adamantyl methacrylate units, which is an alicyclic polymer [Takechi, et al., Journal of Photopolymer Science and Technology, 5(3), 439-446 (1992)], a copolymer containing isobornyl methacrylate units [R. D. Allen, et al., Journal of Photopolymer Science and Technology, 8(4), 623-636 (1995); ibid., 9(3), 465-474 (1996)], and a copolymer containing menthyl methacrylate units [Shida, et al., Journal of Photopolymer Science and Technology, 9(3), 457-464 (1996)].
In the above-exemplified resins, however, adamantyl-containing residue units, isobornyl-containing residue units or menthyl-containing residue unitsxe2x80x94which possess dry etching resistancexe2x80x94do not contain residual groups which can exhibit a difference between the solubility before exposure and that after the exposure. In addition, these alicyclic groups do not contain groups (for example, carboxyl groups) which provide the resins with solubility in an aqueous alkaline solution and also with adhesion to substrates. Accordingly, a homopolymer of a monomer containing an alicyclic group has high hydrophobicity and poor adhesion to substrates under processing (for example, silicon substrates) and can hardly form uniform coating films with good reproducibility. Moreover, due to the lack of any residual groups which make it possible to exhibit a difference between a dissolution rate before exposure and that after the exposure, no pattern can be formed by exposure. The above-described resins can hence be used as resin components in resists only when they are converted into copolymers with a comonomer capable of exhibiting a difference in solubility, such as t-butyl methacrylate or tetrahydropyranyl methacrylate, or with a comonomer capable of imparting substrate adhesion such as methacrylic acid. These comonomers have considerably low dry etching resistance and, nonetheless, their contents are required to be as high as about 50 mole %. Accordingly, such copolymers are significantly reduced in dry etching resistance and have little utility as dry-etching-resistant resins.
There is accordingly a strong desire for a new resist resin material, which has high transparency to radiation of 220 nm or shorter, possesses high etching resistance, contains functional groups permitting exhibition of a difference between the solubility before exposure and that after the exposure, permits development in an aqueous alkaline solution after exposure, and is equipped with improved substrate adhesion.
As a novel resin capable of satisfying these requirements, the present inventors already developed the resin disclosed in JP Kokai 8-259626. A further improvement in dry etching resistance is however desired.
An object of the present invention is therefore to provide a chemically amplified photoresist compositionxe2x80x94which is suitable for use in lithography making use of exposure radiation of 220 nm or shorter, especially of 180 to 220 nm, can exhibit a difference between the solubility before exposure and that after the exposure, permits development in an aqueous alkaline solution after exposure, possesses high substrate adhesion, has high transparency to radiation of 220 nm or shorter, and is equipped with improved etching resistancexe2x80x94and also to provide a pattern forming process making use of the photoresist composition.
The present invention thus relates to a (meth)acrylate represented by the following formula (1): 
wherein R1 represents a hydrogen atom or a methyl group, R2 represents a C17-23 divalent hydrocarbon group containing a bridged cyclic hydrocarbon group, and R3 represents an acid-decomposable group or a hydrogen atom.
This invention also pertains to a polymer available by homopolymerization of the (meth)acrylate represented by the formula (1) or copolymerization of the (meth)acrylate with another copolymerizable compound and having a weight average molecular weight of from 1,000 to 500,000.
This invention is also concerned with a polymer represented by the following formula (2): 
wherein R4, R6 and R9 each represents a hydrogen atom or a methyl group, R5 and R7 each represents a C17-23 divalent hydrocarbon group containing a bridged cyclic hydrocarbon group, R8 represents an acid-decomposable group, R10 represents a hydrogen atom or a C1-12 hydrocarbon group, x+y+z equals to 1, and x, y and z stand for 0 to 1, 0 to 1, and 0 to 0.9, respectively, and having a weight average molecular weight of from 1,000 to 500,000.
This invention also relates to a photoresist composition comprising 70 to 99.8 wt. % of one of the above-described polymers of the present invention and 0.2 to 30 wt. % of a photoacid generator capable of generating an acid upon exposure to radiation.
According to the present invention, it is possible to provide a photoresist material, which can exhibit a difference between the solubility before exposure and that after the exposure, permits development in an aqueous alkaline solution after exposure, possesses high substrate adhesion, has high transparency to radiation of 220 nm or shorter, and is equipped with excellent etching resistance, thereby making it possible to form submicrometer patterns by ArF eximer laser beam in the fabrication of semiconductor devices.
The present invention will hereinafter be described in detail by its embodiments.
In the formula (1), R1 represents a hydrogen atom or a methyl group, and R2 represents a C17-23 divalent hydrocarbon group containing a bridged cyclic hydrocarbon group. Specific examples of R2 can include hexacyclo[6.6.1.13,6.110,13.02,7.09,14]heptadecanediyl, methylhexacyclo[6.6.1.13,6.110,13.02,7.09,14]heptadecanediyl, octacyclo[8.8.0.12,9.14,7.111,18.113,16. 03,8.012,17]docosanediyl, methyloctacyclo[8.8.0.12,9.14,7.111,18.113,16.03,8.012,17]docosanediyl groups, as shown in Table 1.
R3 represents a hydrogen atom or an acid-decomposable group. Specific examples can include t-butyl, tetrahydropyran-2-yl, tetrahydrofuran-2-yl, 4-methoxytetrahydropyran-4-yl, 1-ethoxyethyl, 1-butoxy-ethyl, 1-propoxyethyl, and 3-oxocyclohexyl groups.
As an example of polymers available by homopolymerization of the (meth)acrylate represented by the formula (1) or copolymerization of the (meth)acrylate with another copolymerizable compound, the polymer represented by the formula (2) can be mentioned.
In the formula (2), R4, R6 and R9 each represents a hydrogen atom or a methyl group. R5 and R7 each represents a C17-23 divalent hydrocarbon group containing a bridged cyclic hydrocarbon group. Specific examples can include hexacyclo[6.6.1.13,6.110,13.02,7.09,14]heptadecanediyl, methylhexacyclo[6.6.1.13,6.110,13.02,7.09,14]heptadecanediyl, octacyclo[8.8.0.12,9.14,7.111,18.113,16.03,8.012,17]docosanediyl and methyloctacyclo[8.8.0.12,9.14,7.111,18.113,16.03,8.012,17]docosanediyl groups, as shown in Table 1.
R8 represents an acid-decomposable group. Specific examples can include t-butyl, tetrahydropyran-2-yl, tetrahydrofuran-2-yl, 4-methoxytetrahydropyran-4-yl, 1-ethoxyethyl, 1-butoxyethyl, 1-propoxyethyl, and 3-oxocyclohexyl groups.
R10 represents a hydrogen atom or a C1-12 hydrocarbon group. Specific examples can include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, cyclohexyl, dimethylcyclohexyl, tricyclo[5.2.1.02,6]decyl, norbonyl, adamantyl, and isobornyl groups.
Among (meth)acrylates represented by the formula (1), a vinyl monomer of the formula (1) in which R1 is a methyl group, R2 is a hexacyclo[6.6.1.13,6.110,13.02,7.09,14]heptadecanediyl group, and R3 is a tetrahydropyran-2-yl group can be synthesized, for example, as will be described hereinafter.
First, 8-methoxycarbonyltetracyclo[4.4.0.12,5.17,10]-3-dodecene and dicyclopentadiene are reacted at 170 to 180xc2x0 C. for 17 hours, whereby methoxycarbonylhexacyclo[6.6.1.13,6.110,13.02,7.09,14]heptadecene is obtained.
The methoxycarbonylhexacyclo[6.6.1.13,6.110,13.02,7.09,14]heptadecene is next subjected to alkaline hydrolysis, followed by the protection of the carboxyl group with a tetrahydropyranyl group so that tetrahydropyranyloxycarbonylhexacyclo[6.6.1.13,6.110,13.02,7.09,14] is obtained.
Further, one hydroxyl group is introduced per molecule by using a boran-tetrahydrofuran complex, followed by the reaction with methacryloyl chloride, whereby the target methacrylate is obtained.
The polymer, which is available by homopolymerization of the (meth)acrylate represented by the formula (1) or copolymerization of the (meth)acrylate represented by the formula (1) with another copolymerizable compound, can be obtained by a conventional polymerization process such as radical polymerization or ionic polymerization. This polymerization can be conducted, for example, by stirring the (meth)acrylate by itself or in combination with the another copolymerizable monomer under heat at 50 to 70xc2x0 C. for 0.5 to 12 hours in dry tetrahydrofuran under an atmosphere of an inert gas (argon, nitrogen or the like) in the presence of a suitable radical polymerization initiator [for example, azobisisobutyronitrile (AIBN); monomer/initiator molar ratio: 8 to 200] added therein.
Further, the weight average molecular weight of the polymer according to the present invention ranges from 1,000 to 500,000, with 5,000 to 200,000 being more preferred. A copolymer having desired composition, molecular weight and the like can be obtained by choosing the charge ratio of monomers for the copolymer and other polymerization conditions.
The photoacid generator, which is an essential element of each photoresist composition according to the present invention, may desirably be a photochemical acid generator which generates an acid upon exposure to radiation of 400 nm or shorter, preferably radiation in a range of from 180 nm to 220 nm. Any photoacid generator is usable, insofar as its mixture with the above-described high molecular compound or the like in the present invention is sufficiently soluble in an organic solvent and the resulting solution can be formed into a uniform coating film by a film-forming method such as spin coating. Such photochemical acid generators can be used either singly or in combination.
Examples of photoacid generators usable in the present invention can include the triphenylsulfonium salt derivatives described by J. V. Crivello et al. in Journal of the Organic Chemistry, 43(15), 3055-3058 (1978), onium salts including the triphenylsulfonium salt derivatives as typical examples (for example, compounds such as sulfonium salts, iodonium salts, phosphonium salts, diazonium salts and ammonium salts), 2,6-dinitrobenzyl esters [O. Nalamasu, et al., SPIE Proceedings, 1262, 32 (1990)], 1,2,3-tri(methane-sulfonyloxy)benzene (Takumi Ueno, et al., Proceedings of PME ""89, 413-424, Kodansha Ltd. (1990)3, and sulfo-succimides disclosed in JP Kokai 5-134416.
The content of the photoacid generator is generally from 0.2 to 30 parts by weight, preferably from 1 to 15 parts by weight per 100 parts of the whole components including itself. A content smaller than 0.2 part by weight may lead to a significant reduction in sensitivity, thereby making it difficult to form patterns. On the other hand, a content greater than 30 parts by weight may develop problems such that the formation of a uniform coating film becomes difficult and a residue (scam) tends to occur after development.
The content of the polymer is generally from 70 to 99.8 parts by weight, preferably 85 to 99 parts by weight per 100 parts by weight of the whole components including itself.
Any solvent can be used suitably in the present invention, insofar as it is an organic solvent capable of achieving sufficient dissolution of the polymer and the photoacid generator and the resulting solution can be formed into a uniform coating film by a coating method such as spin coating. Such solvents can be used either singly or in combination. Specific examples can include, but are not limited to, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, tert-butyl alcohol, methylcellosolve acetate, ethylcellosolve acetate, propylene glycol monoethyl ether acetate, methyl lactate, ethyl lactate, 2-methoxybutyl acetate, 2-ethoxyethyl acetate, methyl pyruvate, ethyl pyruvate, methyl 3-methoxypropionate, ethyl 3-methoxypropionate, N-methyl-2-pyrrolidinone, cyclohexanone, cyclopentanone, cyclohexanol, methyl ethyl ketone, 1,4-dioxane, ethylene glycol monomethyl ether, ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether, ethylene glycol monoisopropyl ether, diethylene glycol monomethyl ether, and diethylene glycol dimethyl ether.
Although the xe2x80x9cbasicxe2x80x9d components of the photoresist composition of the present invention are the above-described photoacid generator, resin and solvent, one or more of other components such as dissolution inhibitors, surfactant, pigments, stabilizers, coating property improvers and dyes may also be added as needed.
Each photoresist composition according to the present invention is usable as a new photoresist material having high transparency to radiation of 220 nm or shorter, high dry etching resistance and improved substrate adhesion. Use of the photoresist composition according to the present invention in lithography making use of far ultraviolet radiation of 220 nm or shorter as exposure radiation makes it possible to form submicrometer patterns.
The present invention also relates to process for forming a pattern, which comprises the following steps: coating a substrate with the photoresist composition of the present invention, exposing the thus-coated photoresist composition to radiation having a wavelength of from 180 to 220 nm, baking the thus-exposed photoresist composition, and then developing the thus-baked photoresist composition. In this process, prebaking may be conducted after the coating step if desired. Further, the exposure radiation may desirably be ArF eximer laser.
The present invention will hereinafter be described further by Examples. It is however to be noted that the present invention is not limited to or by them.